1. Field of the Invention
The present invention is generally related to digital pathology.
2. Related Art
The Digital Imaging and Communication in Medicine (“DICOM”) standard is maintained by the National Electronic Manufacturer's Association (“NEMA”), and is supported by large image management systems called Picture Archive and Communication Systems (“PACS”). PACS systems are used in hospitals and labs to store, archive, retrieve, search, and manage images used for clinical and research purposes in medicine, most typically for Radiology images such as radiography (e.g., X-Rays), computed tomography (“CT”) scans, positron emission tomography (“PET”), and magnetic resonance imaging (“MRI”), but also for other modalities such as Ultrasonography, Cardiology, Endoscopy, and Mammography. A large number of clinical and laboratory instruments support DICOM-standard messaging as a means to communicate image information and store it in PACS systems.
The field of pathology is undergoing a transformation in which digital imaging is becoming increasingly important. This transformation is fueled by the commercial availability of instruments for digitizing microscope slides, such as the Aperio ScanScope® described in U.S. Pat. No. 6,711,283 which is incorporated herein by reference in its entirety. The whole-slide images (“WSI” or “digital slides”) made by digitizing microscope slides at diagnostic resolution are very large. Frequently multiple images are created at varying resolutions to facilitate rapid panning and zooming for pathologists. The set of images comprising a single scanned digital slide are typically stored as a single Tagged Image File Format (“TIFF”) file. (TIFF is an open-source standard.) In addition to the size of digital slides, the access characteristics of these images differ from other images presently stored in PACS systems. Pathologists need the ability to rapidly pan and zoom when viewing images.
Unfortunately, there are several limitations of the DICOM standard which impact storage of digital slides. These limitations include DICOM's use of signed 16-bit integers to store the pixel dimensions of images, therefore the maximum image dimensions which can be stored are 32K×32K pixels. This is considerably smaller than a typical digital slide image, and two orders of magnitude smaller than an extremely large digital slide image. Additionally, DICOM uses signed 32-bit integers to store the object size of images, therefore the maximum compressed size of an image is 2 GB. In actual practice many PACS systems are not capable of handling individual images this large; because these PACS systems frequently decompress image data in memory, thereby limiting the maximum uncompressed size of images to 2 GB, they restrict the limit on compressed image size to something considerably smaller. Additionally, while DICOM provides the capability of accessing individual images in a series, as well as individual images in an entire series or entire study, it does not provide the capability of accessing subregions of an individual image. As noted above, the capability to access subregions is important to provide rapid panning and zooming.
Furthermore, the DICOM standard does not make provision for large two-dimensional images such as the digital slides being created for pathology, nor does it incorporate a way to handle images that are logically divided into sub-regions, nor does it incorporate a way to handle multiple images at varying resolutions. The process for evolving the DICOM standard is well-defined but slow moving, and even after the standard is enhanced to support digital slides it will be years before PACS and instrument vendors implement the enhanced standard. In the meantime, a means of using the currently implemented DICOM standard for storing digital slides is needed. Therefore, what is needed is a system and method that facilitates use of the currently implemented DICOM standard for storing and viewing digital slides.